Force measurement of bimetallic thermal disc

ABSTRACT

An apparatus and method for determining the actuation energy generated by a bimetallic actuator during transit between first and second states of stability. The apparatus and method further determining the threshold or set-point temperature of the bimetallic actuator during transit between bi-stable states. Accordingly, the apparatus and method directly measure both the snap force F and the set-point temperature of the bimetallic actuator during transit.

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/241,485, filed in the names of George D. Davisand Robert F. Jordan on Oct. 18, 2000, the complete disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to methods formanufacturing thermally responsive bimetallic members, and in particularto methods for determining the snap energy generated by snap-actionbimetallic members during transit between first and second states ofstability.

BACKGROUND OF THE INVENTION

[0003] Thermally responsive bimetallic members that exhibit asnap-action response are commonly utilized to actuate overheatprotection and thermostatic switching mechanisms. One type of suchmechanisms is a thermostatic switch that utilizes an actuator formed ofa bimetallic material having materials of relatively low and highthermal expansion coefficients joined together along a common interface.Snap-action bimetallic switching mechanisms typically exhibit two statesof stability with each of these states being responsive to apredetermined threshold or set-point temperature. When the switchingmechanism senses a temperature that is below a first lower of thesepredetermined set-point temperatures, the thermally responsive member isin one of the two stable states. Accordingly, when the sensedtemperature is above a second higher predetermined set-pointtemperature, the thermally responsive member snaps to a second of thetwo stable states and remains in this second state while the sensedtemperature remains at or above this second higher set-pointtemperature. Should the sensed temperature be reduced to the first lowertemperature, the temperature of the member is lowered correspondingly.As a result, the thermally responsive member snaps back to the firstlower temperature state. The difference between the two predeterminedset-point temperatures corresponding to the respective first and secondstates of stability is known as the “differential temperature” of thethermally responsive member.

[0004] A known method of manufacturing thermally responsive snap-actionswitches of the variety described above has included a forming operationin which a pre-sized blank of the thermally responsive bimetallic memberis positioned between two opposingly positioned shaping or die members.The shaping members are actuated to engage the bimetallic member,thereby providing the bimetallic member with the desired configurationneeded to achieve snap-action at each of the two predetermined set-pointtemperatures. Such a configuration usually consists of a knee and/orcorresponding bowed portion, a dimpled portion or portions, or a seriesof ridges. Examples of such of formations are described in U.S. Pat. No.3,748,888 and U.S. Pat. No. 3,933,022, each of which is incorporatedherein by reference in its entirety, wherein a thermally responsivesnap-action bimetallic disc is provided.

[0005] U.S. Pat. No. 3,748,888 also describes a smoothly formed priorart disc-shaped snap-action bimetallic member, as illustrated in sideview in FIG. 1. A bimetallic member 1 is formed using a disc of materialformed of two materials 2, 3 having different thermal expansioncoefficients joined together along contiguous surfaces. One of themembers 2 is formed of a material having a relatively high coefficientor rate of thermal expansion, while the other member 3 is formed of amaterial having a low coefficient or rate of thermal expansion relativeto that of the first member 2. The difference in thermal expansioncoefficients between the two members 2, 3 is a factor in determining theset-point temperature at which the resulting bimetallic disc actuator 1operates and in an actuation force F produced by the snap-action. Thedisc-shaped bimetallic member 1 is often circular and, in someinstances, is provided with a small, centrally located aperturetherethrough (not shown). Bimetallic discs of this type are generallyformed by “bumping” a flat circular disc blank with a punch-and-die setto stretch the bimetallic material of the disc into the concavestructure having a depth H1, as illustrated by full line 4 in FIG. 1.The bimetallic disc 1 is formed, for example, with a substantiallyplanar peripheral hoop portion 5 surrounding a central portion 6stretched into a concave configuration. The central portion 6 is mobilerelative to the peripheral hoop portion 5, the central portion 6 movingfrom one side of the peripheral hoop portion 5 to the other as afunction of temperature. The set-point operation temperature and theforce F applied by the snap-action are thus physical characteristics ofthe two members 2, 3 that form the bimetallic member 1.

[0006] Generally, when the bimetallic disc 1 is intended to operate at atemperature above ambient temperature, the disc 1 is bumped on the highexpansion rate side 2 to form the central stretched portion 6, wherebythe central portion 6 is stretched to space the inner concave surfacethereof to a depth H1 away from the plane P of the peripheral hoopportion 5, as illustrated by the full line configuration 4. The depth ofpenetration of the punch during the bumping operation determines thedepth H1 and thus is another factor in determining both the upperset-point temperature and the force F applied by the snap-actionoperation of the disc 1. The set-point operation temperature and theforce F applied by the snap-action are thus also structuralcharacteristics of the bimetallic member 1, as also described inabove-incorporated U.S. Pat. No. 3,748,888.

[0007] The bimetallic disc 1 is illustrated in FIG. 1 in full line 4 inone of its two states of stability. Assuming the bimetallic disc 1 isintended for operation at a set-point temperature above ambienttemperature, the high expansion rate side is located on the surface 2and the low expansion rate side is along the surface 3. If thebimetallic disc 1 is intended for operation at a set-point temperaturebelow ambient temperature, the bimetallic disc 1 is formed in theopposite shape with the low expansion side located on the surface 2 andthe high expansion rate side along the surface 3. For purposes ofexplanation only, the bimetallic disc 1 shown in FIG. 1 is assumed to beintended for operation at a set-point temperature above ambienttemperature. Accordingly, at a temperature well below the upperset-point temperature the bimetallic disc 1 is configured with thecentral stretched portion 6 in an upwardly concave state, as shown bythe upper dotted line 7.

[0008] As the temperature of the bimetallic disc 1 is raised to approachits upper set-point operating temperature, the high expansion ratematerial 2 begins to stretch, while the lower expansion rate material 3remains relatively stable. As the high expansion rate material 2 expandsor grows, it is restrained by the relatively more slowly changing lowerexpansion rate material 3. Both the higher and lower expansion ratesides 2, 3 of the bimetallic disc 1 become distorted by the thermallyinduced stresses, and the central mobile portion 6 of the bimetallicdisc 1 changes configuration with a slow movement or “creep” action fromthe upper dotted line configuration 7 to the full line configuration 4.The inner concave surface of the central mobile portion 6 is spaced thedepth H1 away from the plane P of the peripheral hoop portion 5. Thefull line configuration 4 is considered herein to be a first state ofstability.

[0009] As soon as the temperature of the bimetallic disc 1 reaches itsupper predetermined set-point temperature of operation, the centralstretched portion 6 of the disc 1 moves with snap-action downwardthrough the unstretched hoop portion 5 to the second state of stabilitywith the inner concave surface of the central mobile portion 6 spaced adistance H2 away from the plane P of the peripheral hoop portion 5, asshown by the phantom line 8. If the temperature of the bimetallic disc 1is raised to a still higher temperature, the high expansion ratematerial 2 continues to expand at a greater rate than the relativelylower expansion rate material 3 joined thereto. As a result of thiscontinued differential expansion, the central mobile portion 6 of thebimetallic disc 1 continues to creep toward a state of even greaterdownward concavity, as shown by the second lower dotted lineconfiguration 9.

[0010] As the temperature of the bimetallic disc member 1 is reducedform the high temperature toward the lower predetermined set-pointtemperature of operation, the central mobile portion 6 of the bimetallicdisc 1 moves from the state of extreme concavity, as shown by the lowerdotted line 9, toward the second state of stability indicated in phantom8. As the temperature of the bimetallic disc 1 is reduced below thesecond or lower predetermined set-point temperature of operation, thematerial 2 having the relatively larger thermal coefficient alsocontracts or shrinks more rapidly than the other material 3 having therelatively smaller thermal coefficient. The bimetallic disc 1 changesconfiguration with a similar slow movement or creep action from thestate of greatest downward concavity toward the second state ofstability indicated in phantom 8. As the bimetallic disc 1 reaches thelower set-point temperature, the central stretched portion 6 snaps backthrough the unstretched hoop portion to the first state of stability, asshown by the upper full line 4. If the temperature is decreased stillfurther, the differential expansion between the high and low ratematerials 2, 3 causes the central mobile portion 6 to continue to creeptoward the state of greatest upward concavity, as shown by the upperdotted line 7.

[0011] Many thermal switch designs use one of the bimetallic discs 1that snap into a different state of concavity at a predeterminedthreshold or set-point temperature, thereby closing a contact or otherindicator to signal that the set-point has been reached. The speed atwhich the bimetallic disc actuator 1 changes state is commonly known asthe “snap rate.” As the term implies, the change from one bi-stablestate to the other is not normally instantaneous, but is measurable. Aslow snap rate means that the state change occurs at a low rate ofspeed, while a fast snap rate means that the state change occurs at ahigh rate of speed. Accordingly, in some known configurations of switchand indicator devices, a slow snap rate results in arcing between theoperative electrical contacts. Slow snap rates thus limit the currentcarrying capacity of the thermal switch or indicator device. Incontrast, a fast snap rate means that the change in state occursrapidly, which increases the amount of current the thermal switch orindicator device can carry without arcing. The temperature rate ofchange affects the snap rate. A slower temperature rate of change tendsto slow the snap rate, while a faster temperature rate of change usuallyresults in a faster snap rate. While some applications provide fasttemperature rates, switches and indicators experience very slowtemperature rates in many other applications. In some applications, thetemperature rates may be as low as about 1 degree F. per minute or less.For long-term reliability the device must operate in these very slowtemperature application rates without arcing.

[0012] Furthermore, a minimum force F is required to actuate thecontacts. As described above, the force F is thermally induced in thebimetallic disc 1 as the result of both the depth H1 of the concavityformed in the disc 1, and the differential thermal expansion ratebetween the high and low expansion rate sides 2, 3 thereof. The force Fproduced during transit from one state of stability to the other statemust be sufficiently powerful to overcome the contact restoring force inorder to actuate the device. For example, the force F must be sufficientto overcome a restoring spring force in a flexible switch contact. If abimetallic disc 1 with insufficient snap force F is installed into athermal switch or other indicator device, the switch or device may failprematurely, requiring replacement of the bimetal disc 1 or replacementof the entire mechanism.

[0013] Typically, the snap force F generated by the individualbimetallic disc 1 is tested prior to installation in the using device.For example, the bimetallic discs 1 are pre-tested under maximum load toensure that each exerts sufficient snap force F at temperatureapplication rates of about 1 degree F. per minute or less to actuate thedevice's contact without arcing. One known method of ensuring the snapquality of the bimetallic disc 1 is testing of the force F producedduring actuation of the snap in situ. Pre-testing is thus accomplishedby placing the disc 1 in the intended device and testing the fullyassembled thermal switch or other indicator mechanism. Pre-testing isthus cumbersome and time consuming. Furthermore, the present in situtesting process is typically a simple go/no-go test in which marginallyperforming bimetallic discs 1 may remain undiscovered. The manufacturermay thus be forced to employ excessively conservative quality controlmeasures.

SUMMARY OF THE INVENTION

[0014] The present invention is a method and means for determining anamount of energy released by a thermally responsive snap-actionbimetallic actuator. The method of the invention includes forming abimetallic disc having a mobile center portion surrounded by asubstantially immobile peripheral portion; qualifying an energy releasedby transit of the mobile portion from a first side of the peripheralportion to a second opposite side of the peripheral portion duringoperation of a snap action; and subsequently assembling the disc intooperative relationship with a movable indicator portion of a sensingdevice.

[0015] According to another aspect of the invention, the method of theinvention includes presenting a thermally responsive snap-actionbimetallic actuator to a sensing portion of a force sensing device whilethe actuator is configured in a first pre-snap state wherein a mobileportion of the actuator is spaced away from the sensing portion of theforce sensing device, and determining a force generated by the actuatorduring transit to a second post-snap state wherein the mobile portion ofthe actuator is moved into forceful contact with the sensing portion ofthe force sensing device.

[0016] According to one aspect of the invention, presenting the actuatorto the sensing portion of the force sensing device includes thermallyactivating the actuator to transit to the second post-snap state.

[0017] According to another aspect of the invention, presenting theactuator to the sensing portion of the force sensing device includesplacing the actuator on a support structure configured to support theactuator.

[0018] According to another aspect of the invention, determining a forcegenerated by the actuator includes detecting a peak force generated bymoving the mobile portion of the actuator into forceful contact with thesensing portion of the force sensing device.

[0019] According to another aspect of the invention, presenting theactuator to the sensing portion of the force sensing device includespositioning the actuator in proximity to a thermal stage, and activatingthe thermal stage. Activating the thermal stage includes activating thethermal stage in a controlled manner. According to another aspect of theinvention, determining a force generated by the actuator includesdetermining an energy-temperature rate relationship exhibited by theactuator.

[0020] According to still another aspect of the invention, the method ofthe invention also includes assembling the actuator into operativerelationship with a movable indicator portion of a thermal sensingdevice.

[0021] According to other aspects of the invention, the inventionprovides an energy measuring device having a means for supporting abimetallic member in a first pre-snap state; a means for qualifying anenergy released by the bimetallic member during transit from the firstpre-snap state to a second post-snap state, the qualifying means beingpositioned relative to the supporting means to be engaged by thebimetallic member in the second post-snap state; and a means forthermally activating the bimetallic member, the thermally activatingmeans being positioned relative to the supporting means for thermallyactivating the bimetallic member to transit from the first pre-snapstate to the second post-snap state.

[0022] According to another aspect of the invention, the means forqualifying the released energy includes means for measuring a forcegenerated by the bimetallic member, and may also include means formeasuring a peak force generated by the bimetallic member during thetransit from the first pre-snap state to the second post-snap state.

[0023] According to another aspect of the invention, the thermallyactivating means of the device includes means for thermally activatingthe bimetallic member in a controlled manner, including for example,means for heating or cooling the bimetallic member at a controlled rateof temperature change.

[0024] According to another aspect of the invention, the means forsupporting the bimetallic member in the first pre-snap state includesmeans structured to support a substantially immobile peripheral portionof the bimetallic member while a substantially mobile portion of thebimetallic member that is located centrally to the peripheral portion isdisengaged from the qualifying means.

BRIEF DESCRIPTION OF THE FIGURES

[0025] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0026]FIG. 1 illustrates a known bimetallic actuator disc;

[0027]FIG. 2 is a top plan view of the thermally responsive device ofthe present invention embodied as a snap-action thermal switch;

[0028]FIG. 3 is a cross-sectional view of the snap-action thermal switchillustrated in FIG. 2, wherein the electrical contacts form a closedcircuit;

[0029]FIG. 4 is another cross-sectional view of the snap-action thermalswitch illustrated in FIG. 2, wherein the electrical contacts form anopen circuit;

[0030]FIG. 5 illustrates the thermally responsive bimetallic memberrealized by the method of the invention embodied as a bimetallic discactuator;

[0031]FIG. 6 illustrates the testing apparatus of the invention embodiedas a disc snap-energy tester;

[0032]FIG. 7 illustrates the sizing of an intermediary drive pinpositioned between a sensitive operational portion of a force indicatorof the testing apparatus of the invention shown in FIG. 6 and anactuated bimetallic disc that is configured in a second post-snap state;and

[0033]FIG. 8 illustrates the disc snap-energy tester of the inventionembodied without an intermediary drive pin.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] In the Figures, like numerals indicate like elements.

[0035] The present invention is an apparatus and method for determiningthe snap energy or snap force F generated by a bimetallic actuatorduring transit between first and second states of stability. Theinvention further provides an apparatus and method for determining ofthe threshold or set-point temperature of the bimetallic actuator duringtransit between bi-stable states. Accordingly, the apparatus and methodprovide for directly measuring both the snap force F and the set-pointtemperature of the bimetallic actuator during transit.

[0036]FIG. 2 is a top plan view and FIGS. 3 and 4 are cross-sectionalviews of the thermally responsive device of the present inventionembodied as a snap-action thermal switch 10. The snap-action is drivenby a thermally responsive snap-action actuator of the present inventionembodied as a snap-action bimetallic disc actuator 12, wherein thebimetallic disc actuator 12 includes a minimum snap force F generatedduring transit between bi-stable states at a predetermined set-pointtemperature as determined according to the method of the invention. Forexample, the method is operated using the apparatus of the invention.The thermal switch 10 also includes a pair of electrical contacts 14, 16that are relatively movable under the control of the disc actuator 12.The electrical contacts 14, 16 are mounted on the ends of a pair ofspaced-apart, electrically conductive terminal posts 20, 22 that aremounted in a header 24 such that they are electrically isolated from oneanther. For example, terminal posts 20, 22 are mounted in the metallicheader 24 using a glass or epoxy electrical isolator 26.

[0037] As illustrated in FIGS. 3 and 4, the electrical contacts 14, 16are moveable relative to one another between an open state (FIG. 4) anda closed state (FIG. 3). For example, the movable contact 16 is affixedto an electrically conductive carrier 28 that is embodied as an armatureformed of an electrically conductive spring material. The armature 28 isaffixed in turn in a cantilever fashion to the electrically conductiveterminal post 22 such that a spring pressure S of the armature 28operates to bias the movable contact 16 toward the fixed contact 14 tomake electrical contact therewith, as shown in FIG. 3. The electricalcontacts 14, 16 thus provide an electrically conductive path between theterminal posts 20, 22 such that the terminal posts 20, 22 are shortedtogether.

[0038] The disc actuator 12 is spaced away from the header 24 by aspacer ring 30 interfitted with a peripheral groove 32. A cylindricalcase 34 fits over the spacer ring 30, thereby enclosing the terminalposts 20, 22, the electrical contacts 14, 16, and the disc actuator 12.The case 34 includes a base 36 with a pair of annular steps or lands 38and 40 around the interior thereof and spaced above the base 36. Thelower edge of the spacer ring 30 abuts the upper case land 40. Aperipheral edge portion 42 of the disc actuator 12 is captured within anannular groove created between the lower end of the spacer ring 30 andthe lower case land 38. The disc actuator 12 operates the armaturespring 28 to separate the contacts 14, 16 through the distal end 44 ofan intermediary striker pin 46 fixed to the armature spring 28.Separation of the contacts 14 and 16 creates an open circuit condition.

[0039] As shown in FIG. 3, while the thermal switch 10 is maintainedbelow a predetermined set-point temperature, the disc actuator 12 ismaintained in a first state with the bimetallic disc actuator 12withdrawn into a space 47 between the lower case land 38 and the caseend 36. In this first state, an inner concave surface 48 of an arcuateor dish-shaped central mobile portion 49 of the bimetallic disc actuator12 is spaced away from the intermediary striker pin 46, whereby theactuator force F is removed from the armature 28. The relativelymoveable electrical contacts 14, 16 are moved together under the springpressure S supplied by the armature 28 and thereby form a closedcircuit. The spacing between the inner concave surface 48 of thebimetallic disc actuator 12 and the distal end 44 of the striker pin 46is sufficient to prevent slight movement of the actuator disc 12effecting contact engagement.

[0040] In FIG. 4, the armature 28 is operated under the control of thebimetallic disc actuator 12, which inverts the central mobile portion 49with a snap-action as a function of a predetermined set-pointtemperature between bi-stable states of opposite concavity. As shown inFIG. 4, in response to an increase in the sensed ambient temperatureabove the predetermined set-point, the central mobile portion 49 invertsin a high speed, forceful snap-action into a loaded relationship withthe electrical contacts 14, 16, whereby the inner concave surface 48 isinverted into an outer convex surface 48 that rapidly engages the distalend 44 of the intermediary striker pin 46. The snap-action of thebimetallic disc actuator 12 operates at the predetermined set-pointtemperature to rapidly generate a force F that is predetermined to besufficient to overcome the spring pressure S of the armature 28.Accordingly, operation of the bimetallic disc actuator 12 flexes themovable contact 16 away from the fixed contact 14. For example, the discactuator 12 operates through the intermediary striker pin 46 fixed tothe armature spring 28 to pivot the armature spring 28 upwardly, therebyseparating the contacts 14, 16. Separation of the contacts 14, 16creates an open circuit condition.

[0041] The snap rate and force F with which the central mobile portion49 of the bimetallic disc actuator 12 changes state determine the speedwith which the contacts 14, 16 are allowed to come together to make thecircuit, or are separated to break the circuit. The make and breakspeeds determine how much current can be carried without undesirablearcing between the contacts 14, 16. Faster make and break speedsincrease the amount of current the thermal switch 10 can carry withoutarcing, and thus increase reliability while extending the useful life ofthe thermal switch 10.

[0042] According to one embodiment of the invention, the bimetallic discactuator 12 is fabricated to transit or snap the central mobile portion49 at a high rate while exerting at least a minimum force F.Accordingly, the snap-action of the bimetallic disc actuator 12 changesstate within about 1 millisecond while exerting sufficient force F toovercome the spring pressure S of the armature 28 to break the circuit.The movable contact 16 is thus flexed away from the fixed contact 14rapidly, so that little arcing occurs. The current carrying capacity ofthe thermal switch 10 is thereby maximized.

[0043] When the ambient temperature sensed by the bimetallic discactuator 12 is reduced below the predetermined set-point, the fast snaprate rapidly returns the central mobile portion 49 to the spaced-away,noninterference relationship with the electrical contacts 14, 16, asshown in FIG. 3. The relatively moveable electrical contacts 14, 16 arerapidly moved together again under the spring pressure S of the armature28. A closed circuit between the two terminal posts 20, 22 is therebyformed. Accordingly, one embodiment of the invention provides asnap-action that changes state of the bimetallic disc actuator 12 withinabout 1 millisecond. The spring pressure S of the armature 28 causes themovable contact to follow the retreating central mobile portion 49 ofthe disc actuator 12. The movable contact 16 is thus flexed into contactwith the fixed contact 14 rapidly, so that arcing is minimized and thecurrent carrying capacity of the thermal switch 10 is maximized.

[0044] The thermal switch 10 is sealed to provide protection fromphysical damage. The thermal switch 10 is optionally hermetically sealedwith a dry Nitrogen gas atmosphere having trace Helium gas to provideleak detection, thereby providing the contacts 14, 16 with a clean, safeoperating environment.

[0045]FIG. 5 illustrates the thermally responsive bimetallic memberrealized by the method of the invention embodied as the bimetallic disc12. The central mobile portion 49 of the bimetallic disc 12 generates apredetermined minimum snap force F at a predetermined set-pointtemperature during temperature application at a predetermined rate asdetermined by a test performed in a prescribed manner according to themethod of the invention. For example, the method is operated using theapparatus of the invention. The bimetallic disc actuator 12 according tothe invention is initially fabricated according to generally knownmethods, as described in connection with FIG. 1. For example, athermally responsive bimetallic material 50, such as ASTM-1, is selectedaccording to known criteria for forming a bimetallic actuator. Suchthermally responsive bimetallic material includes a first metallicmaterial 52 having a first coefficient or rate of thermal expansion anda second metallic material 54 having a second relatively higher rate ofthermal expansion. The first and second metallic materials 52, 54 of thethermally responsive bimetallic material 50 are bonded together alongone contiguous surface 56.

[0046] The bimetallic material 50 is formed into a blank of desiredshape and size. For example, a flat, round disk-shaped blank is formedhaving a diameter D sized to move freely within the annular groovecreated in the thermal switch assembly 10 between the lower end of thespacer ring 30 and the lower case land 38.

[0047] The disk-shaped blank is subjected to a forming or “bumping”operation in which the blank of thermally responsive bimetallic materialis positioned between two opposingly positioned shaping members (notshown). The shaping members are actuated to engage the disk-shaped blankof bimetallic material 50, thereby forming the bimetallic disc 12 withthe central mobile portion 49 having a configuration that achievesforceful snap-action at each of the two predetermined set-pointtemperatures. For example, the disk-shaped blank is placed in a femaledie which supports the blank along its peripheral edge portion 42. Amale punch having a spherical end is pressed against the center of thedisc to stretch the metal and form the central mobile portion 49 havingthe inner dish-shaped concave surface 48. The peripheral edge portion 42either retains its substantially planar initial shape, or is formed bythe shaping members with a substantially planar shape. Examples of suchdish-shaped discs are illustrated in U.S. Pat. Nos. 2,717,936 and2,954,447, each of which is incorporated herein by reference in itsentirety. The formed bimetallic disc may be subsequently subjected to aheat treatment operation in order to achieve forceful snap-action of thecentral mobile portion 49 at each of the two predetermined set-pointtemperatures.

[0048] The method and apparatus of the present invention reduce oreliminate reiterative processes of screening the snap force F byinstalling the bimetallic disc actuators 12 into fully assembled thermalswitches 10 or other thermally responsive indicator mechanisms. Incontrast to current reiterative screening methods, the method andapparatus of the present invention determine the energy, i.e. the snapforce F, generated during the snap-action transit of the central mobileportion 49 of the bimetallic disc 12 before it is assembled into asensor mechanism. Low energy bimetallic discs 12 are identified andremoved from a pool of usable hardware. The method and apparatus of thepresent invention thereby result in predictable product delivery sincethe measurement of the snap force F prevents imbalanced mechanisms fromreaching customers. Improved predictability satisfy customerrequirements for quality and reliability while reducing manufacturingcosts.

[0049]FIG. 6 illustrates the testing apparatus of the invention embodiedas a disc snap-energy tester 60. The snap energy of the central mobileportion 49 of each individual disc 12 is effectively determined bythermally inducing the snap-action state transformation under controlledconditions and measuring the snap force F prior to installation into asensor mechanism.

[0050] According to one embodiment of the invention, the disc energytester 60 is an apparatus having a support structure 62 upon which ameasuring device 64 is mounted. The measuring device 64 includes asupporting means 66 for supporting one of the bimetallic discs 12 in amanner substantially similar to the intended application. For example,the supporting means 66 is embodied as a stand or column formed toresemble a portion of the intended thermal switch 10 or other indicatordevice, such as the cylindrical case 34 shown in FIGS. 3 and 4. Thesupporting means 66 is, for example, embodied having a first annularstep or land 68 around the interior thereof and spaced above a base orlower edge 70 thereof The land 68 is sized to support the peripheraledge portion 42 of the bimetallic disc 12 above the base 70. A suitableforce indicator 72, such as a conventional pressure-sensing transducer,is spaced above the bimetallic disc 12. For example, the force indicator72 is suspended from an arm portion 73 of the measuring device 64overhanging the support structure 62 and the supporting means 66. Theforce indicator 72 is of a type that senses a maximum or peak forceF_(p) of the snap and displays the value of the peak force F_(p) in auseful manner. Optionally, the force indicator 72 is positioned insufficiently close proximity to the bimetallic disc 12 that the snapforce F is measured directly. Alternatively, the force indicator 72 isspaced away from the bimetallic disc 12 and the snap force F is measuredthrough an intermediary member 74.

[0051] A means 76 for thermally inducing the snap-action transformationin a controlled manner is provided. For example, the support structure62 is embodied as containing a thermal stage 78 capable of inducing apredetermined, adjustable and controllable rate of temperature change inthe bimetallic disc 12 under test. The thermal stage 78 includesadjustable and controllable heating sources. For example, the heatingsources are thermoelectric devices such as commercially availableelectrical resistive devices. For actuation temperatures above ambient,controlled cooling is provided by alternate energizing and de-energizingof the controllable heating sources. Alternatively, cooling sources areprovided as a manifold formed in the support structure 62 and filledwith a coolant such as liquid nitrogen (LN2) or carbon dioxide (CO2)derived from a conventional external source (not shown). Temperaturecontrol is provided external to the energy tester 60, for example, by aconventional programmable controller 80 such as are well-known forproviding precise control of thermal application rate and steady-statetemperatures.

[0052] The disc energy tester 60 shown in FIG. 6 is embodied for testingthe bimetallic disc 12 intended to actuate the contacts 14, 16 in athermal switch 10. Accordingly, the supporting means 66 is embodied as ahollowed column structure formed in a tube or sleeve configurationsimilar to the cylindrical case 34 shown in FIGS. 3 and 4. The materialof the hollowed column supporting means 66 is a thin stainless steel,which is either the same or a similar material used to form thecylindrical case 34. The sizes and relative spacings of the land 68around the interior of the hollowed column supporting means 66 and thebase 70 thereof are similar to the land 38 and the base 36,respectively, of the cylindrical case 34. Furthermore, the supportingmeans 66 embodied as a hollowed column structure includes a secondannular step or land 82 around the interior thereof The second land 82is paired with the land 68 and spaced above it relative to the base 70.The pair of annular lands 68, 82 are formed to resemble the pair ofannular lands 38, 40 around the interior of the thermal switch case 34.The spacing between the land 82 and each of the respective land 68 andbase 70 is similar to the spacing between land 40 and the respectiveland 38 and base 36 of the case 34. The supporting means 66 is thussubstantially identical to the support structure provided by theintended device.

[0053] As embodied in FIG. 6, the force indicator 72 is spaced away fromthe bimetallic disc 12. The snap force F is applied to the forceindicator 72 through the intermediary member 74, which is embodied as astiff, lightweight drive pin formed of titanium or another suitablematerial. The intermediary drive pin 74 may be hollowed to reduce itsweight. A lighter weight drive pin 74 has less effect on the measurementof the snap force F. A first end portion 84 of the intermediary pin 74is configured similarly to the striker pin 46 and contacts the innerconcave surface 48 of the bimetallic disc 12 when it is configured in afirst pre-snap state, wherein mobile central portion 49 of thebimetallic disc 12 is withdrawn into a space 85 between the firstannular land 68 and the base 70 of the hollowed column structuredsupporting means 66, its inner concave surface 48 being spaced away fromthe force indicator 72. Alternatively, the first portion 84 of theintermediary pin 74 is spaced slightly away from the surface 48 of thebimetallic disc 12 under test, whereby the weight of the intermediarypin 74 is absent from the bimetallic disc 12 in the first pre-snapstate. Accordingly, the first portion 84 of the intermediary drive pin74 is spaced slightly away from the inner concave surface 48 of thebimetallic disc 12. The intermediary pin 74 is thus positioned similarlyto the striker pin 46 of the thermal switch 10 when the central mobileportion 49 of the bimetallic disc 12 is withdrawn into the space 47between the lower case land 38 and the case end 36 so that the contacts14, 16 are configured in a closed circuit condition.

[0054] A second end portion 86 of the pin 74 is configured in a shapesuitable for striking the force indicator 72. The pin 74 has a lengthsized to substantially but not completely fill the space between thebetween the bimetallic disc 12 and the force indicator 72 when thebimetallic disc 12 is configured in the first pre-snap state having theinner concave surface 48 of its central mobile portion 49 spaced awayfrom the force indicator 72, whereby the snap force F is absent from theforce indicator 72. In other words, the pin 74 is short enough that itdoes not press on the force indicator 72 when the bimetallic disc 12 iswithdrawn into the space 85 between the first annular land 68 and thebase 70 of the hollowed column structured supporting means 66.

[0055]FIG. 7 illustrates that the pin 74 is further sized to driveagainst a sensitive operational portion 88 of the force indicator 72when the bimetallic disc 12 is configured in a second post-snap state,wherein the inner concave surface 48 of the central mobile portion 49 isinverted into an outer convex surface 48 directed toward the forceindicator 72. Accordingly, the intermediary drive pin 74 completelyfills the space between the bimetallic disc 12 and the force indicator72 and transmits the snap force F of the bimetallic disc 12 to thesensitive operational portion 88 of the force indicator 72 for themeasurement thereof. Stated differently, during testing the centralmobile portion 49 of the bimetallic disc 12 is inverted into an outerconvex surface 48 that rapidly engages the first end 84 of theintermediary drive pin 74. The drive pin 74 transmits the energy in thesnap of the bimetallic disc 12 through the second end 86 of the pin 74into the sensitive portion 88 of the force indicator 72.

[0056] In summary, the support means 66 provides the annular land 68 tosupport the bimetallic disc along its peripheral edge 42 and within thespace 85 between the annular land 68 and the base 70 of the hollowedcolumn structure. In its first pre-snap state the central mobile portion49 of the bimetallic disc 12 is withdrawn into the space 85 so that itsinner concave surface 48 is spaced away from the force indicator 72. Thesnap force F is thus absent from the force indicator 72. This firstpre-snap state is consistent with the closed circuit condition of thethermal switch 10 wherein the inner concave surface 48 of the centralmobile portion 49 is spaced away from the intermediary striker pin 46and the actuator force F is removed from the armature 28, which permitsthe contacts 14,16 to close. During testing the bimetallic disc 12 isinverted into its second post-snap state wherein the inner concavesurface 48 of the central mobile portion 49 is inverted into an outerconvex surface 48 that rapidly engages the pin 74 and presses it intothe force indicator 72. This second post-snap state is consistent withthe open circuit condition of the thermal switch 10 wherein the outerconvex surface 48 rapidly engages the intermediary striker pin 46 andthe actuator force F is applied to the armature 28, which forces thecontacts 14, 16 open. The apparatus of the invention thus tests the snapforce F applied by the central mobile portion 49 of the bimetallic disc12 under test during transit from the first pre-snap state to the secondpost-snap state.

[0057] According to one embodiment of the invention, a cylindricalspacer 90 is mounted on the second land 82 formed around the interior ofthe hollowed column structured supporting means 66 adjacent to thebimetallic disc 12. The spacer 90 operates similarly to the spacer ring30 of the thermal switch 10. The spacer 90 cooperates with the firstland 68 to form an annular groove within which the peripheral edgeportion 42 of the bimetallic disc 12 is captured. The spacer 90 is alsoprovided with an aperture 92 sized to slidingly engage the drive pin 74.The aperture 92 provides a track for guidance of the drive pin 74. Whenthe drive pin 74 is installed in the aperture 92 of the cylinder 90, thefirst end 84 contacts the central mobile portion 49 of the bimetallicdisc 12, and the second end 86 is positioned for striking the sensitiveportion 88 of the force indicator 72. According to one embodiment of theinvention, the spacer 90 is formed of a thermally insulating material,such as glass or ceramic.

[0058] Spacing adjusting means 94 are optionally provided for adjustingthe position of the first end 84 of the intermediary drive pin 74 tocompensate for differences in the thickness of the bimetallic disc 12under test. For example, the spacing adjusting means 94 are embodied asshims provided between the cylindrical spacer 90 and the second end 86of the drive pin 74.

[0059]FIG. 8 illustrates the energy tester 60 of the invention embodiedwithout the intermediary drive pin 74. The supporting means 66 isembodied having the first annular step or land 68 around the interiorthereof and spaced above a base or lower edge 70 thereof The land 68 issized to support the peripheral edge portion 42 of the bimetallic disc12 above the base 70. The force indicator 72 is spaced above thesupporting means 66 in sufficiently close proximity to the bimetallicdisc 12 that the snap force F is measured directly, without beingtransmitted through the intermediary drive pin 74. Accordingly, theforce indicator 72 is again suspended from the arm portion 73 of themeasuring device 64 overhanging the support structure 62 and thesupporting means 66. According to the embodiment illustrated in FIG. 8,the arm portion 73 spaces the force indicator 72 in sufficiently closeproximity to the bimetallic disc 12 that the snap force F generated bythe central mobile portion 49 is measured directly.

[0060] Spacing adjusting means 94 are again optionally provided tocompensate for differences in the thickness of the bimetallic disc 12.The adjusting means 94 adjusts the relative spacing between the forceindicator 72 and the bimetallic disc 12 under test. For example, thespacing adjusting means 94 are embodied as shims provided between thebimetallic disc 12 under test and operational portion 88 of the forceindicator 72.

[0061] According to the method of the invention, the bimetallic discs 12are subjected to pressure testing that is performed in a prescribedmanner, whereby the energy-tested bimetallic disc 12 of the invention isformed.

[0062] The bimetallic disc 12 is formed according to conventionalmethods by shaping a flat circular disc with a punch-and-die set tostretch the bimetallic material into the concave structure illustratedby the full line 4 in FIG. 1. The bimetallic disc 12 is formed with asubstantially planar peripheral hoop portion 42 surrounding the centralmobile portion 49 that is stretched into a concave configuration.Examples of such dish-shaped discs are illustrated in U.S. Pat. Nos.2,717,936 and 2,954,447, each of which is incorporated herein byreference in its entirety. The formed bimetallic disc may besubsequently subjected to a heat treatment operation in order to achieveforceful snap-action at each of the two predetermined set-pointtemperatures.

[0063] The dish-shaped bimetallic discs 12 are subjected to thermaltesting, which determines the actuation or set-point temperature of eachindividual disc 12, and the discs 12 are categorized according to apredetermined methodology. For example, the tested discs 12 areseparated by material type into categories defined by low set-pointtemperature ranges of about 1 to 2 degrees Fahrenheit with predetermineddifferential temperatures.

[0064] According to the invention, the categorized bimetallic discs 12are presented to the force indicator 72 according to a prescribed mannerfor determining an amount of energy released by the thermally responsivesnap-action bimetallic disc 12. The method of the invention is embodiedas qualifying an energy released by transit of the mobile portion 49 ofthe bimetallic disc 12 from a first pre-snap side of the peripheral edgeportion 42 to a second opposite side of the peripheral edge portion 42during operation of a snap-action. The qualified bimetallic disc 12 issubsequently assembled into an operative relationship with a movableindicator portion of a thermal sensing device. For example, thequalified disc 12 is assembled with the thermal switch 10 in theposition described above for interacting with the intermediary strikerpin 46, whereby the qualified disc 12 moves the mobile contact 16 awayfrom the fixed contact 14 during actuation.

[0065] The released energy is qualified by presenting the disc 12 to aforce sensing device such as the force indicator 72. The disc 12 ispresented on the supporting means 66 while the central mobile portion 49is positioned on the first pre-snap side of the substantially immobileperipheral edge portion 42 opposite from the force indicator 72. Thedisc 12 is supported on its generally immobile peripheral edge portion42. The dish-shaped central mobile portion 49 is extended on the side ofthe edge portion 42 away from the force indicator 72. In other words,the central mobile portion 49 is withdrawn into the space 85 between thefirst annular land 68 and the base 70 of the hollowed column structuredsupporting means 66, the inner concave surface 48 of the central mobileportion 49 being thus spaced away from the force indicator 72. The disc12 is presented sufficiently closely to the operational portion 88 forceindicator 72 that the mobile portion 49 is positioned to forcefullyinteract with the operational sensing portion 88 of the force indicator72 during transit to the second post-snap side of the peripheral edgeportion 42 proximate to the force indicator 72. As described above, theforceful interaction with the operational sensing portion 88 of theforce indicator 72 is either direct or through an intermediary mechanismsuch as the drive pin 74.

[0066] The snap energy is released by thermally activating thebimetallic disc 12 in the presence of the force indicator 72. The methodthus includes changing the temperature of the disc 12 to transit thecentral mobile portion 49 from the first pre-snap side to the secondpost-snap side of the peripheral edge portion 42 proximate to the forceindicator 72. The mobile portion 49 of the disc 12 is moved into contactwith the operational portion 88 of the force indicator 72 during transitfrom the first pre-snap side to the second post-snap side of theperipheral edge portion 42. To qualify according to the method of theinvention, a bimetallic disc 12 applies a minimum force FM to theoperational portion 88 of the force indicator 72 during actuation. Theoperational sensing portion 88 of the force indicator 72 thus senses thepeak force F_(p) generated by the transit of the central mobile portion49 during actuation.

[0067] The bimetallic disc 12 is thermally activated by either heatingor cooling it through the set-point temperature using the thermal stage78. Heating and cooling are under the control of the programmablecontroller 80. According to one embodiment of the invention, theprogrammable controller 80 is used to control the heating and coolingrates of thermal stage 78 so that the bimetallic disc 12 is heated orcooled at temperature rates as low as about 1 degree F. per minute orless. The disc 12 under test is thus qualified for long-term reliabilityin devices that must operate in conditions of very slow temperatureapplication rates without arcing. Optionally, the controller 80 is usedto control the thermal stage 78 at several different temperature ratesof change so that the disc 12 under test is thermally activated at aplurality of different controlled rates of temperature change. Snapforce F and set-point temperature data are taken at each actuation ofthe disc 12, and an energy-temperature rate relationship is determinedfor the disc 12 under test.

[0068] Obviously, many modifications and variations of the presentinvention are possible in light of the above teachings. Thus, it is tobe understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedabove.

What is claimed is:
 1. A method for determining an amount of energyreleased by a thermally responsive snap-action bimetallic actuator, themethod comprising: presenting a thermally responsive snap-actionbimetallic actuator to a sensing portion of a force sensing device whilethe actuator is configured in a first pre-snap state wherein a mobileportion of the actuator is spaced away from the sensing portion of theforce sensing device; and determining a force generated by the actuatorduring transit to a second post-snap state wherein the mobile portion ofthe actuator is moved into forceful contact with the sensing portion ofthe force sensing device.
 2. The method of claim 1 wherein presentingthe actuator to the sensing portion of the force sensing device includesthermally activating the actuator to transit to the second post-snapstate.
 3. The method of claim 1 wherein presenting the actuator to thesensing portion of the force sensing device includes placing theactuator on a support structure configured to support the actuator. 4.The method of claim 1 wherein determining a force generated by theactuator includes detecting a peak force generated by moving the mobileportion of the actuator into forceful contact with the sensing portionof the force sensing device.
 5. The method of claim 1 wherein presentingthe actuator to the sensing portion of the force sensing device includespositioning the actuator in proximity to a thermal stage and activatingthe thermal stage.
 6. The method of claim 5 wherein activating thethermal stage includes activating the thermal stage in a controlledmanner.
 7. The method of claim 6 wherein determining a force generatedby the actuator includes determining an energy-temperature raterelationship exhibited by the actuator.
 8. The method of claim 7,further comprising assembling the actuator into operative relationshipwith a movable indicator portion of a thermal sensing device.
 9. Amethod for determining a force generated by a thermally responsivesnap-action bimetallic disc during transit between first and secondstates, the method comprising: presenting the thermally responsivesnap-action bimetallic disc to a force indicator on a support structurewhile a mobile portion of the disc is positioned on one side of asubstantially immobile edge portion opposite from the force indicator,the disc being presented sufficiently closely to the force indicatorthat the mobile portion is positioned to forcefully interact with asensing portion of the force indicator during transit to a second sideof the edge portion proximate to the force indicator; changing atemperature of the disc to transit the mobile portion into a position onthe second side of the edge portion proximate to the force indicator;and sensing with the sensing portion of the force indicator a peak forcegenerated by the transit of the mobile portion.
 10. The method of claim9 wherein changing the temperature of the disc includes changing atemperature of the support structure.
 11. The method of claim 9 whereinchanging the temperature of the disc includes changing the temperatureat a controlled rate.
 12. The method of claim 9 wherein the temperatureof the disc is below an actuation temperature of the disc prior tochanging.
 13. The method of claim 12 wherein changing the temperature ofthe disc includes increasing the temperature above the actuationtemperature.
 14. The method of claim 9 wherein presenting the disc tothe force indicator on a support structure includes simulating a portionof a structure intended to support the disc during operation in atemperature sensing device.
 15. The method of claim 9 wherein changing atemperature of the disc to transit the mobile portion into a position onthe second side of the edge portion proximate to the force indicatorincludes generating a force with the mobile portion of the disc.
 16. Themethod of claim 15 wherein sensing a peak force generated by the transitof the mobile portion includes applying the force generated with themobile portion of the disc to the sensing portion of the forceindicator.
 17. A method for determining an amount of energy released bya thermally responsive snap-action bimetallic disc, the methodcomprising: forming a bimetallic disc having a mobile center portionsurrounded by a substantially immobile peripheral portion; qualifying anenergy released by transit of the mobile portion from a first side ofthe peripheral portion to a second opposite side of the peripheralportion during operation of a snap action; and subsequently assemblingthe disc into operative relationship with a movable indicator portion ofa sensing device.
 18. The method of claim 17 wherein qualifying thereleased energy includes thermally activating the disc in the presenceof a force sensing device.
 19. The method of claim 18 wherein qualifyingthe released energy further includes moving the mobile portion of thedisc into contact with an operational portion of the force sensingdevice during transit of the mobile portion from the first side to thesecond side of the peripheral portion.
 20. The method of claim 19wherein thermally activating the disc includes one of heating andcooling the disc.
 21. The method of claim 19 wherein qualifying thereleased energy includes determining a minimum force applied to theoperational portion of the force sensing device during transit of themobile portion of the disc.
 22. The method of claim 19 whereinqualifying the released energy includes thermally activating the disc ata controlled rate of temperature change.
 23. The method of claim 22wherein qualifying the released energy includes thermally activating thedisc at a plurality of different controlled rates of temperature change.24. An energy measuring device comprising: a means for supporting abimetallic member in a first pre-snap state; a means for qualifying anenergy released by the bimetallic member during transit from the firstpre-snap state to a second post-snap state, the qualifying means beingpositioned relative to the supporting means to be engaged by thebimetallic member in the second post-snap state; and a means forthermally activating the bimetallic member, the thermally activatingmeans being positioned relative to the supporting means for thermallyactivating the bimetallic member to transit from the first pre-snapstate to the second post-snap state.
 25. The device of claim 24 whereinthe means for qualifying the released energy includes means formeasuring a force generated by the bimetallic member.
 26. The device ofclaim 25 wherein the means for measuring a force generated by thebimetallic member includes means for measuring a peak force generated bythe bimetallic member during the transit from the first pre-snap stateto the second post-snap state.
 27. The device of claim 24 wherein thethermally activating means includes means for thermally activating thebimetallic member in a controlled manner.
 28. The device of claim 27wherein the means for thermally activating the bimetallic member in acontrolled manner includes means for heating or cooling the bimetallicmember at a controlled rate of temperature change.
 29. The device ofclaim 24 wherein the means for supporting the bimetallic member in thefirst pre-snap state includes means structured to support asubstantially immobile peripheral portion of the bimetallic member whilea substantially mobile portion of the bimetallic member that is locatedcentrally to the peripheral portion is disengaged from the qualifyingmeans.
 30. A bimetallic actuator testing device comprising: a forceindicator; a support structure that is spaced a predetermined distanceaway from the force indicator; and a thermal stage positioned relativeto the support structure for changing a temperature of the supportstructure.
 31. The testing device of claim 30 wherein the supportstructure that is spaced relative to the force indicator such that abimetallic actuator that is configured in a first pre-snap state may beplaced thereon and such that, when actuated to a second post-snap state,the bimetallic actuator contacts the force indicator.
 32. The testingdevice of claim 30 wherein the support structure includes an annularland spaced above a base, the land being sized to support a peripheraledge portion of the bimetallic actuator above the base.
 33. The testingdevice of claim 30, further comprising an intermediary member suspendedbetween the support structure and the force indicator, the intermediarymember being structured to transmit a force generated by the bimetallicactuator to a force sensing surface of the force indicator.
 34. Thetesting device of claim 30 wherein a force sensing portion of the forceindicator is positioned to sense peak force generated by the bimetallicactuator by transiting between the first pre-snap state and the secondpost-snap state.
 35. The testing device of claim 30 wherein the forceindicator is structured to display a value of the peak force.
 36. Thetesting device of claim 30 wherein the force indicator is a conventionalpressure-sensing transducer.
 37. A device for testing a force generatedby transit of a thermally responsive bimetallic disc between a firstpre-snap state and a second post-snap state, the bimetallic disc beingconfigured with a substantially round immobile edge portion positionedperipherally to a mobile center portion that extends on a first side ofthe edge portion when the disc is configured in the first pre-snap stateand transits with a snap-action in response to a predetermined set-pointtemperature through the edge portion to extend on a second side of theedge portion, the testing device comprising: a columnar supportstructure having a first annular support surface sized to support theedge portion positioned peripherally to a mobile center portion of athermally responsive bimetallic disc; a force indicator having a forcesensing surface positioned relative to the support structure to beforcefully engaged by the mobile center portion of the thermallyresponsive bimetallic disc when the mobile center portion transits witha snap-action in response to a predetermined set-point temperaturethrough the edge portion from a first side of the edge portion to extendon a second side of the edge portion; and a thermal stage positionedrelative to the support structure to induce the predetermined set-pointtemperature in the thermally responsive bimetallic disc supported on thesupport structure.
 38. The testing device of claim 37 wherein thethermal stage is positioned adjacent and in close proximity to thesupport structure.
 39. The testing device of claim 38 wherein the forceindicator is suspended opposite the thermal stage.
 40. The testingdevice of claim 37 wherein the force indicator is a pressure-sensingtransducer of a type that is capable of sensing a peak force applied tothe force sensing surface.
 41. The testing device of claim 40 whereinthe pressure-sensing transducer is of a type that is capable ofdisplaying the value of the peak force in a useful manner.
 42. Thetesting device of claim 37, further comprising a drive member positionedintermediately between the annular support surface of the supportstructure and the force sensing surface of the force indicator fortransmitting a force generated when the mobile center portion of thebimetallic disc transits with a snap-action through the edge portionfrom the first side of the edge portion to extend on the second side ofthe edge portion.
 43. The testing device of claim 42 wherein thecolumnar support structure includes a second annular support surfacespaced away from the first annular support surface toward the forcesensing surface of the force indicator and sized having an interiordimension larger than the edge portion of the bimetallic disc; andfurther comprising a spacer that engages the second annular supportsurface and cooperates with the first annular support surface to form anannular groove within which the peripheral edge portion of thebimetallic disc is captured, the spacer including an aperture with whichthe drive member is slidingly engaged for motion between the annularsupport surface of the support structure and the force sensing surfaceof the force indicator.
 44. The testing device of claim 43 wherein thedrive member and the spacer are sized such that a first end portion ofthe drive member is positioned by the spacer to contact an inner concavesurface of the bimetallic disc when the disc is configured in a firstpre-snap state and installed into the annular groove, and a second endportion of the drive member is positioned by the spacer to engage theforce sensing surface of the force indicator when the disc is configuredin a second post-snap state.